1. Field of the Invention
This invention relates to a TV system capable of inserting a pilot carrier to be used as an AGC reference signal in a TV signal and easily taking it out, and more particularly to an improvement of an AGC system for such a television system.
2. Description of the Prior Art
Today, to transmit a TV signal, an optical signal changeable in response to brightness of the picture to be reproduced is first converted to an electric signal (video signal), and a complex video signal made by superposing sync signal B to video signal A as shown in FIG. 1 is actually transmitted. Further, upon transmission, the carrier amplitude is modulated by the complex video signal (hereinafter simply called "video signal"). There are several modulating methods as shown in FIGS. 2 through 4. FIG. 2 shows a so-called negative modulating method wherein the carrier amplitude becomes maximum at the sync signal portion B of the video signal A and minimum at the largest point a (white level) of the video signal A. This is characterized in that the signal power decreases at the white level portion and is widely adopted in Japan and the U.S.A. as a standard method. FIG. 3 shows a so-called positive modulating method wherein, contrary to the method of FIG. 2, the carrier amplitude becomes maximum at the white level portion a of the signal A. This is characterized in that the signal power increases at the white level portion. FIG. 4 shows a so-called sync signal suppression modulating method wherein the sync signal part B falls in the video signal A. This is similar to the method of FIG. 2 in that the signal power decreases at the white level portion.
Pay TV broadcasting permitting only special receivers to reproduce a video signal was recently realized and is now in practice utilizing the above-mentioned TV signal. Such pay TV broadcasting has such a system that the broadcasting station transmits a scrambled signal by scrambling a normal TV signal by an encoder while a subscriber in turn reproduces the given picture after decoding the scrambled TV signal by a decoder of his own.
If the broadcasting station uses a special modulating method, there is a possibility that receivers cannot decode the modulated TV signal nor reproduce the picture. For example, when the broadcasting station transmits a TV signal modulated according to the positive modulating method of FIG. 3 or the sync signal suppression method of FIG. 4, a receiver designed to receive only a TV signal modulated according to the negative modulating method of FIG. 2 cannot properly separate the sync signal, whereby it is impossible to reproduce a normal picture. That is, said purpose for transmitting a signal in secrecy is thus attained.
Now think about an automatic gain control (AGC) function for keeping a signal level always constant in a receiver's system regardless of variation of the received signal input level. Since the negative modulating method keeps sync signal level always constant regardless of video signal level, it makes it possible to easily perform a peak AGC function by detecting peaks of the signal. On the other hand, the positive modulating method or the sync signal suppression modulating method does not permit a peak AGC function because the peaks always vary in response to the video signal level. In this connection, when using the sync signal suppression modulating method, average AGC or manual AGC is adopted. However, neither can realize a satisfying effect.
Beside these, there is a further AGC method, namely, a pilot carrier insertion method which is adopted in the field of microcommunication. FIG. 5 is a spectrum graph of a TV signal band obtained when adopting this AGC method in dealing with a TV signal. The abscissa represents frequency and the ordinate represents signal level. Reference numeral 1 designates a band of a modulated video signal, and 2 is a pilot carrier. According to this, the transmitting station inserts the pilot carrier 2 with a determined frequency adjacent to the video signal band regardless of the waveform of the video signal, while the receiver's system takes out the pilot carrier 2 by means of a bandpass filter. Thus, the pilot carrier is utilized as an AGC reference signal.
However, this method causes an increase of the TV signal band because the pilot signal 2 must be inserted outside the band 1 of the video signal as shown in FIG. 5 in order to avoid interference between the pilot carrier and the video signal. For example, according to NTSC, the bandwidth for a TV signal should be 6 MHz, so that it is sometimes difficult to insert a pilot carrier in a TV signal having no leeway in its signal band without adversely influencing the video signal and the audio signal.
It is therefore desirable, for TV signal reception, to provide an AGC method having a stable function.